Such methods for ascertaining and monitoring fill level in a container are frequently applied in measuring devices of automation- and process control technology. Produced and sold by the assignee, for example, are such fill level measuring devices under the marks, Prosonic, Leveiflex and Micropilot, which work using the travel-time measuring method and serve to determine and/or to monitor fill level of a medium in a container. These fill level measuring devices transmit a periodic transmission signal in the microwaves- or ultrasonic range by means of a transmitting/receiving element in the direction of the surface of a fill substance and receive the reflected echo signals after a distance dependent, travel time. Usual fill level measuring devices working with microwaves can be divided basically into two classes; a first class, in the case of which the microwaves are sent by means of an antenna toward the fill substance, reflected on the surface of the fill substance and then received back after a distance dependent, travel time and a second class, in the case of which the microwaves are led along a waveguide toward the fill substance, reflected on the surface of the fill substance due to the impedance jump existing there and the reflected waves led back along the waveguide.
Formed from the received echo signals, as a rule, is an echo function representing echo amplitude as a function of travel time, wherein each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the transmitting element.
In this ascertained echo function, a wanted echo is determined, which corresponds to the reflection of the transmission signal on the surface of the fill substance. From the travel time of the wanted echo, there results in the case of known propagation velocity of the transmission signals directly the separation between the surface of the fill substance and the transmitting element.
In order to simplify the echo curve evaluation, not the received, raw signal of the pulse sequences is used, but, instead, their envelope, the so called envelope curve, is ascertained. The envelope curve is won, for example, by rectifying the raw signal of the pulse sequences and then filtering through a lowpass filter.
There are a number of different method aspects for determining the wanted echo in an envelope curve, and these can be divided into two basic methods, namely the static detection methods with static echo search algorithms and/or the dynamic detection methods with dynamic echo search algorithms, for example, by applying historical information.
In a first method aspect, using a static echo search method, a static echo search algorithm selects the wanted echo as that which has a larger amplitude than the remaining echoes. Thus, the echo in the envelope curve with the largest amplitude is selected as wanted echo.
In a second method aspect, using a static echo search method, it is assumed by a static echo search algorithm that the wanted echo is the echo occurring first in the envelope curve after the transmission pulse. Thus, the first echo in the envelope curve is selected as wanted echo.
It is possible to combine the two method aspects with one another in a static echo search algorithm by e.g. defining a so-called first echo factor. The first echo factor is a predetermined factor, by which an echo must exceed a certain amplitude, in order to be recognized as a wanted echo. Alternatively, a travel time dependent, echo threshold can be defined, which an echo must exceed, in order to be recognized as wanted echo.
In a third method aspect, the fill-level measuring device is told, one time, the current fill level. The fill-level measuring device can, based on the specified fill level, identify the associated echo as wanted echo and follow such e.g. by a suitable dynamic echo search algorithm. Such methods are referred to as echo tracking. In such case, e.g. in each measuring cycle, maxima of the echo signal or the echo function are determined and, due to the knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific, maximum to be expected rate of change of fill level, the wanted echo is ascertained. From travel time of the so ascertained current wanted echo, there results then the new fill level.
An expanded method for monitoring the rate of change is described approximately in DE 198 245 267 A1, wherein, for individual echoes, a velocity measure is determined, which represents the change of the distance measures of two echoes evoked by the same object as a function of time. The velocity measures of various individual echoes are compared with one another and the result of the comparison is taken into consideration for an echo evaluation. Especially, an individual echo is evaluated with high probability to be a multiecho, when its velocity measure is displayable as a sum of weighted velocity measures of individual echoes with a smaller distance measure.
A fourth method is described in DE 102 60 962 A1. There, the wanted echo is ascertained based on data stored earlier in a memory. In such case, there are derived, from received echo signals, echo functions, which reflect the amplitudes of the echo signals as a function of their travel time. The echo functions are stored in a table, wherein each column serves for accommodating, in each case, one echo function. The echo functions are stored in the columns in a sequence, which correspond to the fill levels associated with the respective echo functions. In operation, the wanted echo and the associated fill level are determined based on the echo function of the current transmission signal with the assistance of the table.
In DE 103 60 710 A1, a fifth method is described, in the case of which periodically transmission signals are sent toward the fill substance, their echo signals recorded and converted into an echo function, at least one echo characteristic of the echo function determined, and, based on the echo characteristics of at least one preceding measurement, a prediction derived for the echo characteristics to be expected in the case of the current measurement. The echo characteristics of the current measurement are determined taking into consideration the prediction, and, based on the echo characteristics, the current fill level is ascertained. In the broadest sense, this method is like an echo tracking.
In DE 10 2004 052 110 A1, a sixth method is described, which achieves improvement of the wanted echo detection by an echo evaluation and classification of the echoes in the envelope curve.
These above described methods work, per se, without problem in a large number of applications. Problems occur, however, always when the echo coming from the fill level cannot be identified based on the method without there being some doubt as to the correctness of the identification due to multiechoes and disturbing echoes.
In the first method aspect, for example, measurement problems occur, in case installed objects present in the container reflect the transmission signals better than the surface of the fill substance.
In the case of the echo tracking according to the third method aspect, measurement problems occur, in case, during operation, the wanted echo runs over a disturbance echo and subsequently the disturbance echo is tracked further as a wrong wanted echo. Furthermore, there is a problem, in case, upon turn-on, the preceding wanted echo signal no longer agrees with the actual wanted echo signal or the preceding wanted echo signal is not known.
If, mistakenly, another echo than the fill-level echo is classified as wanted echo, there is the danger that a wrong fill level is output, without that such is noticed. This can, depending on application, lead to an overfilling of containers, to the running of pumps empty or to other events sometimes associated with considerable danger.
Due to the above described measurement problems, it can come to a wrong or unsettled measured value ascertainment of fill level of the medium in the container. A disturbance echo signal or a multiecho signal can be recognized as wanted echo signal. In the worst case, a so-called echo loss can be experienced, in the case of which the wanted echo signal can no longer be identified, or found.